Art of producing musical sounds electrically



April 7, 1936. G. SMILEY 2,036,892

ART OF PRODUCING MUSICAL SOUNDS ELBCTRICALLY Filed Dec. 4, 1933 2Sheets-Sheet 1 Ill a G P 5 1; 5 n w- -mm Time In Time 1P2 lr ngzNToR BY2 M4... Quil ATTORNEYS Eci Ajaril 7, 1936. 4 G. SMILEY 2,036,892

ART OF PRODUCING MUSICAL SOUNDS ELECTRiCALLY Filed Dec. 4, 1933 2Sheets-Sheet 2 5 g INVENTOR ATTORNEYS Patented Apr. 7, 1936 UNITEDSTATES her or raonucme MUSICAL sounns amc'rmcmr Gilbert Smiley,Brighton, Masm, assignor to Pratt,

Read a company, Deep River. W com- R S S U E D Application December 4,1933, Serial Nil- 700,837

ration of Connecticut Claims.

My present invention has for its object to produce musical soundselectrically. A more specific object of my invention is to provide acircuit arrangement for producing an output wave which 5 possesses theharmonics, required under certain conditions, to render a given notemusically desirable or pleasing to the ear.

Still another object contemplated is the provision of means for addingto a more or less pure 1o sinusoidal wave of given frequency, as derivedfrom an appropriate oscillator or appropriate oscillators, one ormoreadditional harmonics of the fundamental of said wave at a properamplitude, or for the reinforcement of harmonics already present but ofinsuflicient amplitude, by

the use of a vacuum tube repeater from the output of which the resultantwave form may be amplified and reproduced by a loud speaker.

To these and other ends my invention comprises further improvements allof which will be more fully set forth in the following specification andthe novel features pointed out in the accompanying claims.

In the drawings:

Fig. 1 is a diagrammatic illustration of a ,vacuum tube circuit forrepeating audible frequency oscillations and applying thereto thedesired characteristic wave form.

"Figs. 2 to 5 are plate-current curves versus grid-potential curves,illustrating the manner in which invention is accomplished.

Fig. 6 is a schematic illustration of a means for effecting simultaneousadjustments of grid supply-potential and input wave amplitude.

Similar reference characters in the several figures indicate like parts.

In illustrating my invention I have shown a repeater circuit comprisinga vacuum tube V having a plate P, a grid G and a cathode K. The 40 inputcircuit, across which the incoming oscillations are impressed, comprisesthe conductors l and 2, the former leading through a blocking condenserCo to one end of a voltage-divider 3 the slider 3a of which is connectedby a conductor 5 4 to the grid G, and the latter leading directly to thecathode K. The other end of the divider 3 is connected to the cathode bya conductor 5 in which a biasing source of potential Ec for the grid isprovided. The output circuit comprises the plate and cathode leads 6 andI, which may be connected to an amplifier 8 in turn feeding into aloudspeaker 9, as will be understood. The plate lead 6 has a blockingcondenser Cp connected therein. The plate-cathode circuit comprises thesource Eb for supplying the operating potential to the plate P and theresistance R through which said potential is supplied and which servesas a coupling resistance in the output circuit. The condensers Co and Cpprevent the operating D. C. potentials from being transmitted throughthe input and output terminals to the instrumentalities connectedthereto.

Any convenient source or sources of oscillations feed into the inputconductors l and 2. In Fig. 1 I have shown a single source S connectedto these conductors. In my development of musical instruments for theplaying of chords, however, I employ a series of independent ordependently related sources of oscillations, such as vacuum tubeoscillatory circuits, tuned to the various frequencies called for by thevarious keys of the keyboard. I'hus several or all of these oscillatorycircuits may be adapted to feed into the repeater circuit shown in Fig.1, each oscillatory circuit so connected being arranged to supply aproper proportion of its output when the key associated with thefrequency or note which it produces is struck or depressed. If desired,a separate repeater circuit may nevertheless be provided for each of thesources, each repeater circuit receiving the proper intensity of outputfrom its associated source when the latters key is depressed. Fortheproduction of single notes, the repeater circuit of Fig. 1 may beassociated with a single oscillator, the frequency of which iscontrollable by suitable means cooperating with the keyboard.

I have found in practice that in many "of the types of oscillators orgenerators of audible frequency voltage waves, the wave form produced istoo pure, 1. e., too close to a sinusoidal wave to create variousdesirable musical effects. In other words, such a wave form does notreproduce a musical note acceptable for many purposes, as it is lackingin timbre.

My invention contemplates broadly introducing an oscillatory wave of apure or insufficiently impure form into a repeater circuit and thereintroducing the necessary harmonics to produce what is known in amusical sense as overtones, the final output from the repeater being awave form which, by proper control, is caused to render a note similaror identicalto that sounded by an instrument it may be desired tosimulate, or, moreover. to create discrete tonal effects, notnecessarily imitative of existing instruments, but none the lesspleasing and desirable. In general this is accomplished by theintroduction in the repeater circuit of such controlling elements thatthe harmonic content of the output wave may be widely varied to permitdesired and desirable tonal effects to be attained.

The object desired may be accomplished by controlling both the grid biasof the repeater tube V and the amplitude of the inputwave and takingadvantage of the repeater tube characteristics. The grid bias may becontrolled by proper adjustment of the potential of the supply Ec, andthe amplitude of the input wave may be controlled by proper adjustmentof the slider 3a on the voltage-divider 3.

The curve I shown in Figs. 2 to 5 inclusive illustrates theplate-current characteristic of a. repeater tube V such as I may employin accordance with my invention. In Fig. 2 this tube is operated as alinear repeater, in which case the output wave, which corresponds to theplate current wave I2, is an amplified facsimile of the grid potentialwave I I produced by the influence of the incoming fluctuating wave. Inoperating the repeater circuit in this instance the grid bias potentialis selected, as indicated by the axis Eel, to fall at about the centerof the straight portion of the plate current" characteristic curve Hi.This bias is such that equal grid potential changes, within certainlimits, produce equal plate current changes about the steady platecurrent, indicated by the axis Ipl, resulting under the steady biasconditions. Thus, practically no distortion results when alternatingpotential of amplitude Al is applied to the grid G, causing an outputwave of double amplitude B between crests to be established about theplate axis Ipl.

A comparison of the figure just described with Fig. 3 shows howharmonics of appreciable magnitude may be introduced in the-output waveby increasing the grid operating potential in a negative direction. Theamplitude of the grid potential wave is at the same time slightlyincreased to the value A2, which 'is done to maintain the desired doubleamplitude B of the output wave in the plate circuit. As seen, the inputwave I3 is established along the new axis E02, which results in shiftingthe plate axis to 1122 along which the output wave I4 is correspondinglydistorted from its former approximately sinusoidal form shown in Fig. 2.p

In Fig. 4 I have illustrated the bias of the grid of the repeater tubeadjusted to place the input axis at the cut-off point E03. of the platecurrent characteristic curve It). In this case the amplitude of theinput voltage must be considerably increased, as its new value A3indicates, in order to maintain the output corresponding to afluctuating flow of current in the plate circuit at the same amplitudeas before. As observed, the output wave 96 established along the newaxis Ip3 as a result of the grid potential fluctuations i5 is in thisinstance definitely distorted, its wave form introducing such harmonicsas to adapt it for certain musical purposes.

5 shows a further adjustment of the grid bias to a point E04 beyond thecut-oil point of the tube and it is seen that, in order to continue theoutput at the same double amplitude B, it is necessary to still furtherincrease the amplitude of the input wave, as shown by A4. The input waveis indicated by IT and, as observed, the form of the correspondingoutput wave l8 es- .tablished along the axis Ip4 departs to a stillgreater degree from that supplied to the repeater input, plate currentpassing only during the crests of the more positive half cycles of theinput wave.

From the above description it is apparent that the present inventionprovides the distinct advantage that yarious musical efi'ects as totimbre or tone quality of the notes produced may be obtained by properadjustment of the operating parameters of the tube V. Satisfactorycontrol of the resultant wave form supplied to the amplifier 8 and loudspeaker 9 is in this manner eiiected without the use or variable filtermeans otherwise necessary for such control, were a sutflciency ofharmonics in the first place available. In practice, the operatingparameters of the tube may be made variable, either continuously or inpredeterminedsteps, between conditions for maximum linearity or minimumdistortion and conditions for any selected maximum of distortion. Ineither case, the amplitude of the input wave and the operating potentialat the grid may be adjustable simultaneously to maintain their properrelationship for an essentially constant apparent volume output. Anarrangement ior such simultaneous adjustment is shown schematically inFig. 6. A voltage-divided 20 is connected across the source E0 and itsslider 20a is operated conjointly with the slider 30. of the divider 3,as by a common handle 2!. If desired, tle amplifier 8 may includesuitable filter means for the elimination of undesirable orover-amplitude harmonics or their attenuation to negligible or desirableamplitude in the manner known in the art. parameters of the tube may beselected to produce desired harmonics of a desired amplitude andthereafter made permanent. In this instance a filter means is providedfor controlling the harmonic content.

It is understood that, if desired, such values 0! operating potential Ecmay be provided at the rid of the tube V as to cause the input potentialchanges to swing through the upper portions 10a (Fig. 2) of thecharacteristic curve Hi. The distorting effects which are obtained inthis manner are similar to those which have been described. In thisinstance the more the grid supply-potential is increased in a positivedirection, the richer is the harmonic content of the output wave. It is,of course, understood that the more the grid supply-potential isincreased in a positive direction, the greater the amplitude to whichthe input wave should be adjusted to maintain the intensity of theoutput wave substantially constant. However, this distortion is much thesame as will be realized by operation on the low plate current end ofthe curve l0, and has the disadvantage that, in some instances, the gridof the tube V will draw current placing an appreciable load on theoscillatory source, which may, in some instances, destroy stability offundamental frequency. Further, in drawing current, the grid mayintroduce certain high order harmonics which are usually musicallyundesirable. For these reasons it is generally preferable to operate thegrid in the more negative region, though the invention comprises allpossibilities of deliberate distortion in repeater circuits.

Although the invention is illustrated in the drawings and describedherein in connection with a single repeater tube, it is understood thatcircuits involving a plurality of repeater tubes, each of which isoperated in a. manner as described, may be used to produce otherdesirable musical efiects. For example, a pair of non-linearly operatedrepeater tubes may be used in a pushpull arrangement, i. e.,symmetrically disposed in a back-to-back circuit connection. Assuming Itis understood that the operating aoaasaa 3 the required balance betweensuch tubes, no even harmonics will be found to exist in the output waveother than those already present in the input wave.

What is claimed is:

1. In a musical instrument, the combination with means for producingelectrical oscillations of audible frequency, of a harmonic introducingthermionic tube repeater circuit to which said oscillations aresupplied, said repeater circuit comprising means for varying its gridoperating potential to vary the harmonic content of its outputoscillations, said repeater circuit also comprising means for varyingthe amplitude of the input oscillations at its grid to maintain theintensity of its output oscillations substantially constant.

2. In a musical instrument, the combination with means for producing anaudible frequency electrical wave of substantially pure sinusoidal form,of a wave distorting thermionic tube repeater circuit to which said waveis supplied, said repeater circuit comprising means for varying its gridsupply-potential to vary the form of its output wave, said repeatercircuit also comprising meansfor varying the amplitude of the input waveat its grid to maintain the intensity of its output wave substantiallyconstant.

3. In a musical instrument, the combination with means for producingelectrical oscillations of audible frequency, of a harmonic introducingthermionic tube repeater circuit to which said oscillations aresupplied, said repeater circuit comprising means for varying its gridsupplypotential to vary the harmonic content of its output oscillations,said repeater circuit also comprising means for varying the amplitude ofthe input oscillations at its grid to maintainthe intensity of itsoutput oscillations substantially constant, -said supply-potentialvarying means in one of its positions of adjustment effecting a biasvalue at cut-off or beyond causing maximum introduction of harmonics.

4. In a musical instrument, the combination with means for producing anaudible frequency electrical wave of substantially pure sinusoidal form,of a wave distorting thermionic tube repeater circuit to which said waveis supplied, said repeater circuit comprising means for varying its gridsupply-potential to vary the form of its output wave, said repeatercircuit also comprising means for varying the amplitude of the inputwave at its grid to maintain the intensity of its outputwavesubstantially constant, said supply-potential varying means in oneof its positions of adjustment effecting a bias value at cut-off orbeyond causing maximum wave distortion.

5. In a musical instrument, the combination with means for producingelectrical oscillations .of audible frequency, of a harmonic introducingsimultaneously.

eflecting harmonic introduction between conditions for substantialabsence of harmonic introduction and conditions for a predeterminedmaximum of harmonic introduction.

6. In a musical instrument, the combination with means for producing anaudible frequency electrical wave of substantially pure sinusoidal form,of a wave distorting thermionic tube repeater circuit to which said waveis supplied, said repeater circuit comprising means for varying its gridsupply-potential to vary the form of its output wave, said repeatercircuit also comprising means for varying the amplitude of the inputwave at its grid to maintain the intensity of its output wavesubstantially constant, said supply-potential varying means effectingwave distortion between conditions for substantial absence' of wavedistortion and conditions for a predetermined maximum wave distortion.

'7. In a musical instrument, the combination with means for producingelectrical oscillations of audible frequency, of a harmonic introducingthermionic tube repeater circuit to which said oscillations aresupplied, said repeater circuit comprising means for varying its gridoperating potential, to vary the harmonic content of its outputoscillations, said repeater circuit also comprising means simultaneouslyoperated with said operating potential varying means for varying theamplitude of the input oscillations at its grid to maintain theintensity of its output oscillations substantially constant. I

8. In a musical instrument, the combination with means for producing anaudible frequency electrical wave of substantially pure sinusoidal form,of a wave distorting thermionic tube repeater circuit to which said waveissupplied, said repeater circuit comprising means for varying its gridsupply-potential to vary the form of its output wave, said thermioniccircuit also comprising means simultaneously operated with saidsupply-potential varying means for varying the amplitude of the inputwave at its grid to maintain the intensity of its output wavesubstantially constant.

9. In a musical instrument, the combination with means for producingelectrical oscillations of audible frequency, of a harmonic introducinthermionic tube repeater circuit tov which said oscillations aresupplied, said repeater circuit comprising an adjustable source ofgrid-supply potential for varying the harmonic content of its outputoscillations and a voltage-divider for varying the amplitude of theinput oscillations at its grid to maintain the intensity of its outputoscillations substantially constant.

10. In a musical instrument, the combination with means for producingelectrical oscillations of audible frequency, of a harmonic introducingthermionic tube repeater circuit to which said oscillations aresupplied, said repeater circuit comprising an adjustable source ofgrid-supply potential for varying the harmonic content of its outputoscillations and a voltage-divider for varying the amplitude of theinput oscillations at its grid to maintain the intensity of its outputoscillations substantially constant, said voltagedivider and saidadjustable source being operated GILBERT SMILEY.

